Frequently, large structures such as offshore platforms, are anchored to the earth with tubular piles. Those piles are inserted through structural members of the platform and driven into the earth. They are then attached or "grouted" to the structural member. Often, these piles extend several hundred feet into the earth.
The length, number and size of the pilings are in large part determined by the type of soil through which the piles are driven. Characteristics of the soil are normally determined before fabrication of the structure by analysis of soil samples or by other means. Unfortunately, soil characteristics are sometimes inaccurately predicted and after the structure is installed it is found that the as-driven piles provide inadequate support.
On other occasions, the soil qualities can be accurately determined but it may be desirable to enhance the loading capability of the piling. For example, a particularly buoyant structure such as a tension leg platform may require enhanced pull-out strength on one or more piles in soils of varying resistance properties. In still other cases, the structure is installed in permafrost and the frozen condition of the soil must be maintained to prevent settling of the structure.
Various methods of increasing or maintaining the load bearing capacity of piles have been developed. For example, "anchor bumps" can be created on the pile to increase the load carrying capacity and pull out resistance of the pile (U.S. Pat. No. 3,995,438). In some cases this may, however, not sufficiently increase the capacity of the pile.
Methods of maintaining the frozen condition of the soil have also been described (French Pat. No. 475,226, see also U.S. Pat. No. 4,111,258). These methods rely on the circulation of cold ambient air through the pile to maintain a permafrost condition. Such methods could not be applied in an area where extremely cold ambient conditions do not exist for a substantial portion of the year. Further, they provide only for maintenance of the frozen condition of soil to prevent subsidence and do not provide increased pull-out capacity.
Soil has also been frozen in order to provide temporary structural support while installing a subterranean tunnel as well as to prevent settling of a runway set in permafrost and prevent water encroachment during the installation of a ventilation shaft (Braun, B., and Nash, W. R., "Ground Freezing for Construction," Civil Engineering, January, 1985, pp. 54-56). In none of these situations is a long-term method of substantially increasing the load bearing and pull-out capacity of a tubular pile provided.
In summary, it is clear that an improved method of substantially enhancing the load bearing and pull-out capacity of piles is desirable.